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Tachometer Output is putting out an incorrect signal?

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    Tachometer Output is putting out an incorrect signal?

    Hello everyone, I have an early WhodWho MS unit and everything was fine running wasted spark and suddenly one day the tachometer died. Long story short I bought an oscilloscope and the pulse looked fine so I proceeded to buy a new cluster and one of those SI board capacitor replacements despite my board metering okay. Still no dice. Finally I spoke to Greg from Bavarian Restorations and he told me that my signal voltages were incorrect. I’m at a loss with this car, sadly I only have the one so I don’t have another to test this stuff on. Thanks for any insight.

    #2
    The MS Tach output jumper is supposed to be a 12V square wave output, so +12V pulled low (to zero). Is this "whodwho MS unit" microsquirt or megasquirt based? I'm just guessing it's microsquirt based. You are probing your tach out pin and referencing that oscilloscope to the ground plane of the ecu, correct? It looks like you have 10.27V getting pulled down to 7.25V instead of 12V getting pulled down to zero. The microsquirt circuit for tach out is the following:

    Click image for larger version

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    Based on what you've presented me with I'd suspect that Q1 is on the fritz.

    IG @turbovarg
    '91 318is, M20B25 turbo
    [CoTM: 4-18]
    '94 525iT slicktop

    Comment


      #3
      Originally posted by Doorgoo View Post
      I bought an oscilloscope
      Off topic, what oscilloscope is that? are you happy with it?
      sigpic
      1987 - 325i Convertible Delphin Auto [SOLD], 325i Convertible Delphin Manual [SOLD]
      1989 - 325i Convertible Bronzit m30b35 swapped [SCRAPPED], 325i Sedan Alpine Auto[DD]
      1991 - 325i Coupe Laguna Manual [Project], 535i Sedan Alpine [SCRAPPED]

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        #4
        Originally posted by varg View Post
        The MS Tach output jumper is supposed to be a 12V square wave output, so +12V pulled low (to zero). Is this "whodwho MS unit" microsquirt or megasquirt based? I'm just guessing it's microsquirt based. You are probing your tach out pin and referencing that oscilloscope to the ground plane of the ecu, correct? It looks like you have 10.27V getting pulled down to 7.25V instead of 12V getting pulled down to zero. The microsquirt circuit for tach out is the following:

        Click image for larger version

Name:	Annotation 2020-07-21 180801.jpg
Views:	306
Size:	8.1 KB
ID:	9940141
        Based on what you've presented me with I'd suspect that Q1 is on the fritz.
        I will take a look at that thank you! It is indeed microsquirt based

        Comment


          #5
          Originally posted by McGyver View Post

          Off topic, what oscilloscope is that? are you happy with it?
          It’s one of those DIY kits you can get everywhere, the DSO 138. It’s definitely hobbyist grade, I only bought this because I can’t get to my work one due to COVID. It’ll get the job done but it’s less than intuitive and the signal won’t be completely pure but it clearly was enough to reveal my issue!

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            #6
            varg do you think it’s worth trying to fix or am I better off buying a new microsquirt module? Like obviously a component is cheaper but am I just asking for more trouble down the line
            Last edited by Doorgoo; 07-22-2020, 03:16 PM.

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              #7
              I don't see any reason to replace the whole unit, the failure could be a fluke or it could have been caused by something like a wiring problem or internal short that damaged the transistor. That's if it's the transistor, it's just my guess based on what was presented. In any case inspect it and probe the InjLed pin, that should be logic level with the opposite waveform to the tachout waveform, also unplug the cluster and test the resistance between the tach out pin and ground. It should be open.

              IG @turbovarg
              '91 318is, M20B25 turbo
              [CoTM: 4-18]
              '94 525iT slicktop

              Comment


                #8
                varg you were right you beautiful bastard! It works again I could cry

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